Multilateration is a navigation technique that utilizes measurement of the difference in distance to one or more emitters (stations) at known locations that broadcast signals at known times. However, this measurement of the difference in distance yields an infinite number of locations that satisfy the measurement. A hyperbolic curve is formed when these possible locations are plotted. A second measurement needs to be taken from a different pair of emitters (stations) to produce a second curve which intersects with the first curve and thus locate the exact location along the hyperbolic curve. When the two or more curves are compared, a small number of possible locations are identified to obtain a “fix”. Multilateration is a common technique in radio navigation systems.
A line of bearing (LOB) measurement is based on a direction from a point to a target (emitter). An estimate of a target's location can be found by combining two different LOB measurements to obtain a fix. The segment between the two LOB sensors is referred to as the “baseline,” and the distance to the target from the center of the baseline is the “standoff distance.”
A relatively accurate measure of the physical relationship between the sensors and the target is the angle formed by the rays connecting the target to each sensor. For targets at ranges in excess of one baseline, the angle descriptor (which decreases) more accurately describes the off-boresight case than the range-to-baseline ratio (which does not decrease). However, measurement errors of the target location become increasingly substantial, when the angle formed by the line segments connecting the sensors to the target is small. For example, if the estimate of a target's location is performed by an airborne platform, such as an aircraft, the platform (aircraft) has to be in a straight and level flight for the estimate to be accurate. This imposes a significant limitation of the aircraft and the environment, in which the target position is to be determined.
The shortcomings of the LOB algorithm have been somewhat addressed by using measurements made only during straight and level flight and a fixed standoff from the target. When fixes are calculated using data obtained under these constraints, the geo-fix may have a sufficient accuracy. However, the drawbacks are that the aircraft maneuvering must be tightly controlled when a geo-location fix is being attempted, and that measurements are dropped if the constraints of straight and level flight (for example, during turns) and/or standoff distance are not satisfied.
These drawbacks restrict how and when a conventional standoff RF geo-location system can be used. A common operational scenario is that when a target is detected, an initial geo-fix is computed, and the operator points a camera toward the target. Putting a camera on the target often requires turning and flying the aircraft closer toward the target. When this happens the aircraft is no longer flying broadside to the emitter in straight and level flight, so LOB measurements begin to exhibit the spirograph pattern.
A recent development in airborne geo-location is the introduction of near vertical direction finding (NVDF) systems. Unlike standoff direction finding (DF) systems, an NVDF system is looking down (as the name implies) from an aircraft, not sideways. Since the aircraft is looking down, NVDF has a much smaller field of view than a standoff system. On the other hand, NVDF systems provide two angles of arrival, and can provide instantaneous position estimates. Instantaneous measurements enable an NVDF system to track moving emitters. Since the operation and capabilities of NVDF and conventional standoff are quite different, they complement, rather than replace, one another.